This invention relates to a helium cooled superconducting magnet assembly suitable for magnetic resonance imaging (hereinafter called "MRI") utilizing a mechanical cryocooler and recondenser for recondensing the resultant helium gas back into liquid helium, and more particularly to improved efficiency and simplified gaskets for thermally connecting the cryocooler to the recondenser of the superconducting magnet.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing a cryogen such as liquid helium. The extreme cold maintains current flow through the magnet coils after a power source initially connected to the coil (for a relatively short period) is disconnected due to the absence of electrical resistance in the cold magnet coils, thereby maintaining a strong magnetic field. Superconducting magnet assemblies find wide application in the field of MRI.
The provision and storing of a steady supply of liquid helium to MRI installations all over the world has proved to be difficult and costly leading to considerable research and development efforts directed at minimizing the need to replenish the boiling liquid helium such as by recondensing the resultant helium gas.
Superconducting magnets which recondense the helium gas back to liquid helium are often referred to as zero boiloff (ZBO) magnets. The helium gas formed by the boiling of liquid helium in the superconducting magnet helium pressure vessel is flowed through passageways in the recondenser cooled by the cryocooler to recondense the helium gas back to liquid helium for return to the liquid helium bath in the pressure vessel. The efficient thermal coupling of the mechanical refrigerator or cryocooler to the recondenser is extremely important because the cryocooler cooling capacity and operational limits are often approached in a ZBO superconducting magnet, taxing the thermal ability of the system to provide the necessary cooling for recondensing the helium gas. In addition, it is necessary to accomplish this while facilitating insertion and adjustment of the cryocooler in the superconducting magnet assembly without damaging the cryocooler by exerting excessive pressure on the cryocooler to obtain the efficient thermal coupling required in such a system.
U.S. Pat. No. 5,701,742, issued Dec. 30, 1997 and assigned to the assignee of the subject patent, discloses the use of a deformable indium gasket for the thermal coupling interface to decrease the coupling pressure required. However, it has been found necessary in some ZBO superconducting magnets to further increase the thermal efficiency to ensure adequate cooling because of marginal cooling capability of some ZBO magnet assemblies while further minimizing coupling pressure to avoid possible damage to the cryocooler. This invention thus constitutes an improvement over that of the aforementioned U.S. Pat. No. 5,701,742 invention.
Indium, while soft and pliable at room temperatures, has proven to be extremely hard and difficult to properly compress when at superconducting temperatures. Slight imperfections and variations in thickness have required so much pressure on the gasket for good thermal conductance as to strip the threads of adjustment screws or damage the cryocooler housing. Obtaining uniform optimum thermal conductance across the thermal interface gasket with a minimum applied pressure has often been difficult or elusive.
As a result, it becomes extremely important to provide an improved yet uncomplex efficient thermal coupling between the cryocooler and recondenser to enable efficient recondensing of the helium gas back to liquid helium in a ZBO superconducting magnet.